J. Am. Chem. SOC. 1995,117, 11025-11026
Mechanism of Lithium Cuprate Conjugate Addition: Neutral Tetracoordinate CUI Cuprates as Essential Intermediates
11025
Scheme la
James P. Snyder i
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OR a) 0,Lit(OH2) b) 0-CH,
Cherry L. Emerson Center for Scientij5c Computation, Department of Chemistry Emory University, Atlanta, Georgia 30329 Received June 6, 1995 The initial discovery of cuprate conjugate addition was complemented by a mechanistic study ruling out the operation of a cyclic six-centered transition state.' In the intervening 30 years, cuprate-facilitated carbon-carbon bond formation has blossomed as a fundamental tool for the synthetic chemist.2Full mechanistic understanding of the reaction, however, has yet to be achieved. Early arguments that electron transfer guides the conjugate alkylation3 continue to receive some a t t e n t i ~ n . ~ However, the bulk of the mechanistic work has suggested the reaction to be initiated by a lithiated e n ~ n e , ~ a copper ~ , ~ , olefin ~~ n - c ~ m p l e x ,an ~ , a-cuprioketone,7b ~ a charge transfer complex,8 a cyclic t e m ~ l a t e , ~or~ ,some ~ , ~ combination of these along a closed-shell pathway. Spectroscopic studies have detected various species by stopped-flow m e t h o d ~ l o g yor~ by ~ operating at low tem~eratures.63~~ Subsequent steps are believed to involve an as-yet unobserved ''Cu1I1 " intermediate,6c,8s9although the latter has been recognized as a high-energy species.I0 RlCuLi
I
-CUR
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In the present study, we examine a series of structures which model key parts of the potential energy surface for the conjugate addition of dimethyl cuprate to acrolein. Particular emphasis is placed on how the n-complexes are converted to P-alkylated product and on the question of the intermediacy of Cu"'. All structures have been optimized without symmetry using density (1) House, H. 0.;Respess, W. L.; Whitesides, G. M. J. Org. Chem. 1966, 31, 3128.
(2) (a) Posner, G. H. An Introduction to Synthesis Using Organocopper Reagents; John Wiley and Sons: New York, 1980. (b) Rossiter, B. E.; Swingle, N. M. Chem. Rev. 1992, 92, 77. Lipshutz, B. H.; Sengupta, S. Org. React. 1992, 41, 135. (3) (a) House, H. 0.;Umen, M. J. J. Am. Chem. SOC. 1972, 94, 5495. (b) House, H. 0. Acc. Chem. Res. 1976, 9, 59. (4) Bertz, S. H.; Dabbagh, G.; Mujsce, A. M. J. Am. Chem. SOC. 1991, 113, 631. (5) (a) House, H. 0.; Chu, C.-Y. J. Org. Chem. 1976, 41, 3083. (b) Krause, N. J: Org. Chem. 1992, 57, 3509. (6) (a) Riviere, H.; Tang, P. W. C. R. Acad. Sci. Paris 1972, C-274; Bull. SOC. Chim. Fr. 1973, 2455. (b) Berlan, J.; Koosha, K.; Battioni, J. P. Bull. SOC. Chim. Fr II 1978, 575. Berlan, J.; Battioni, J. P.; Koosha, K. Bull SOC. Chim. Fr. II 1979, 183. (c) Krauss, S. R.; Smith, S. G. J. Am. Chem. SOC. 1981, 103, 141. (d) Snyder, J. P.; Tipsword, G. E.; Spangler, D. P. J. Am. Chem. SOC. 1992, 114, 1507. (e) Lipshutz, B. H.; Dimock, S. H.; James, B. J. Am. Chem. SOC. 1993, 115, 9283. (7) (a) Christenson, B.; Ullenius, C. Pure Appl. Chem. 1988,60,57. (b) Bergdahl, M.; Eriksson, M.; Nilsson, M.; Olsson, T. J. Org. Chem. 1993, 58, 7238. (c) Bertz, S. H.; Smith. R. A. J. J. Am. Chem. SOC. 1989, 111, 8276. (d) Alexakis, A.; Sedrani, R.; Mangeney, P. Tetrahedron Lett. 1990, 31, 345. (e) Krause, N.; Wagner, R.; Gerold, A. J. Am. Chem. SOC. 1994, 116, 381. (0 Vellekoop, A. S.; Smith, R. A. J. J. Am. Chem. SOC. 1994, 116, 2902. (8) Smith, R. A. J.; Hannah, D. J. Tetrahedron 1979, 101, 4236. (9) (a) Johnson, C. R.; Dutra, G. A. J. Am. Chem. SOC.1973, 95, 7783. (b) Casey, C. P.; Cesa, M. C. J. Am. Chem. SOC.1979,101,4236. (c) Corey, E. J.; Boaz, N. W. Tetrahedron Lett. 1984, 25, 3063; 1985, 26, 6015. (d) Keegstra, M. A. Tetrahedron 1992, 48, 2690. (e) Dorigo, A. E.; Wanner, J.; Schleyer, P. v. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 476. (lO)Collman, J. P.; Hegedus, L. S. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1980.
functional theory (DFT)," the D95V valence double-5 basis, and the Hay-Wadt 10-electron relativistic effective core potential on copper (B3LYP/LANL2DZ).'2,13 Key structures have been further subjected to a fixed-point calculation at the MP2/HW3 level of theoryI4 and analyzed within the natural population analysis (NPA) f r a m e ~ 0 r k . I ~ Scheme 1 depicts three routes to products stemming from n-complex 1.16 The latter carries a molecule of water at lithium as a first solvation shell surrogate for ether solvent and is characterized by a nearly square-planar arrangement around the metal with all Cu-C bond lengths within 2.08-2.14 A. An effort was made to locate the rearranged allylic complex 2a, suggested to be capable of reversible complexation with enone and instrumental in directing reaction s t e r e o c h e m i ~ t r y .All ~~~~~ starting points led to the lithium-isomeric n-olefin complex 3, more stable than 1 by 3.6 kcdmol. Carbonyl methylation to give 2b (R' = CH3) did, however, lead to a stationary point. This suggests that if complex 1 is sufficiently long-lived, it might be possible to capture the allyl complex as a stable intermediate. Superficially, the calculated result complies with the postul a t i ~ nand ~ ~i ~, ~o l~a t i o n ~ ~of, 'enol ~ ~ -TMS ' ~ ethers when cuprate addition is carried out in the presence of MesSiCl. In fact, recent (1 1) (a) Johnson, B. G.; Gill, P. M.; Pople, J. A. J. Chem. Phys. 1993, 98, 5612. (b) Andzelm, J.; Wimmer, E. J. Chem. Phys. 1992, 96, 1280. (c) Becke, A. D. J. Chem. Phys. 1993, 98,5648. (d) Ricca, A.; Bauschlicher, C. W. J. Phvs. Chem. 1994. 98. 12899. (12) Absilute and zero-point (ZPE, kcal/mol) energies (in au) at the G92/ DFTI3-optimizedB3LYPkANL2DZ geometries: 1, -55 1.858 543 (99.4 kcaymol); 2b, -509.710031; 3, -551.864 312; 4b, -315.794 473 (66.9); 5b, -315.794 358 (66.9); 6b, -470.822 665 (119.3); 7b, -470.809 557 (118.7); 8, -391.082 042 (73.4); CHsCH3, -79.818 862 (47.2); MepO, -155.005 480 (50.0). (13) For basis set details and the performance of B3LYPLANL2DZ for CH3Cu and [CH~CUCH~I-, see: Snyder. J. P.; Bertz, S. H. J. Org. Chem. 1995, 60, 4312. (14) Jonas, V.; Frenking, G.; Reetz, M. T. J. Comput. Chem. 1992, 13 (8), 919. Bohme, M.; Frenking, G.; Reetz, M. T. Organometallics 1994, 13, 4237. (15) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. Reed, A. E.; Curtis, L. A.; Weinhold, F. Chem. Rev., 1988, 88, 899.
(16) Previous cuprate enone n-complex calculations include refs 6a, 7d, and 13. See also: Dorigo, A. E.; Morokuma, K. J. Chem. SOC., Chem. Commun. 1989, 1884. (17) (a) Corey, E. J.; Hannon, F. J.; Boaz, N. W. Tetrahedron 1989,45, 545. (b) For a counterview, see: Horiguchi, Y.; Komatsu, M.; Kuwajima, I. Tetrahedron Lett. 1989, 30, 7087. (18) Alexakis, A,; Berlan, J.; Besace, Y. Tetrahedron Lett. 1986, 27, 1047. Horiguchi, Y.; Matsuzawa, S.; Nakamura, E.; Kuwajima, I. Tetrahedron Lett. 1986,27,4025. Nakamura, E.; Matsusawa, S.; Horiguchi, Y.; Kuwajima, I. Tetrahedron Lett. 1986,27,4029. Johnson, C. R.; Marren, T. J. Tetrahedron Lett. 1987,28,27. Bertz, S . H.; Smith, R. A. J. Tetrahedron 1990, 46, 409 1.
0002-7863/95/1517-11025$09.00/00 1995 American Chemical Society
11026 J. Am. Chem. SOC., Vol. 117, No. 44, 1995
Communications to the Editor
,,...'CHI
MelO
1J H3C-CH3
CH,
kb
+
(hChO-CU-CH3
8
Figure 1. Relative B3LYP/LANL2DZ//T33LYP/LANL2DZ (MP2/ HW3//B3LYP/LANL2DZ) energies, kcal/mol, including B3LYPI LANL2DZ zero-point energies for the transformation of 6b to 4b and CH3CH3 8.
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experiments demonstrate that the silylating agent combines with product enolate only after rate-determining cuprate alkylati~n.'~ Complex 3 can spontaneously ring-open to the putative Cu"' enolate intermediate 4a, followed by reductive elimination through transition state 5a. Since the neutral copper center is insulated by CH2 from the enolate moiety, structure 4 has been modeled as trimethylcopper (4b). The T-shaped moleculez0(no negative eigenvalues of the Hessian matrix) resides in a very shallow local minimum, followed by 5b (one negative eigenvalue) only 0.07 kcal/mol higher in energy (Figure 1). On the MP2/HW3 surface, 5b is less stable than 4b by 1.5 kcal/mol. Similar calculations for RCuH2 (R = H and CH3) are unambiguous in demonstrating that the less-substituted T-structures are transition states between RCueH2 and HCu*RH product complexes.2' Quite clearly, trialkylcopper derivatives have little or no independent existence and, therefore, cannot serve as intermediates in cuprate conjugate addition, reversiblegc or otherwise. The remarkable stability of the [Cu(CF3)4]- anion22suggested to us that solvent might intercept complex 3 to give the neutral higher order copper species 6. Treated as the trimethylcopper dimethyl ether solvate, 6b optimizes without symmetry to give a nearly square-planar structure (Figure 2, supporting information) stable to dissociation to Me20 and 4b by 11.9 and 17.2 kcal/mol at the B3LYPLANL2DZ and MP2/HW3 levels of theory, respectively (Figure 1). Such an intermediate has been proposed previously in connection with cuprate substitution r e a ~ t i o n s . ~ ~ Transition , ~ , ~ ~ , ' ~state 7b yields a single negative eigenvalue, is likewise distorted square-planar, and lies 7.6 and 9.2 kcdmol higher in energy, respectively, than 6b.23 In both 6b and 7b, the plane of the Me20 solvent is perpendicular to the copper-carbon plane (Figure 2). The calculated barrier is in excellent agreement with Krauss and Smith's pseudo-first(19) Bertz, S . H.; Miao, G.; Rossiter, B. E.; Snyder, J. P. J. Am. Chem. SOC. 1995, 117, 11023.
(20) Tamaki, A.; Magennis, S. A,; Kochi, J. K. J. Am. Chem. SOC. 1974, 96, 6140. Komiya, S.; Albright, T. A,; Hoffmann, R.; Kochi, J. K. J. Am. Chem. SOC. 1976, 98, 1255. (21) Snyder, J. P.; Musaev, J.; Morokuma, K., unpublished work. Crump, W. Angew. Chem., (22) Naumann, D.; Roy, T.; Tebbe, K.-F.; Int. Ed. Engl. 1993, 32, 1482.
order decomposition rates ( k = 0.06-3.9 s-l; Ea= 18-21 kcal/ mol), suggested to apply to a prereductive elimination step.7c The result that decomposition of 6b is energetically less expensive than dissociation to solvent and 4b ensures that trialkylcopper is not operating by the dissociative reaction channel. The proposed mechanism, then, involves solvent attack on n-complex 3 to give 6, which, unlike square-planar trialkylphosphine gold complexes,20undergoes reductive elimination without prior dissociation to a T-shaped trialkyl metal species, i.e., 4. With respect to the oxidation state of the metal in 4, 6, and the other copper-containing structures in Scheme 1, a comparative NPA analysis reveals that the 3d metal population ranges from 9.4 to 9.7 electrons as it does for the classical organocopper reagents CuCH3 and [CH~CUCH~I-. All structures, neutral and charged, incorporate a dIo metal with an atomic charge of 5 1.O. They can be regarded as CUIcomplexes whose bonds are dominated by ionic contributions, but whose structures are determined by a nonvanishing degree of covalent C ~ ( d ~ 2 - ~ 2 ) ligand binding. Structures 4 and 6, for example, show a relatively strong covalent Cu-C bond at the leg of the threecarbon T, while the Cu-C arms are similar to the bonds in [CH~CUCH~I-. As for [ C U ( C F ~ ) ~ ]the - , ~electron ~ distributions amount to partial oxidation at the ligand centers. NPA analysis for 4 and 6 posits that the T- and leg-methyls have lost 1.0 and 0.5 electrons, respectively, relative to CH3-. The intimate interplay between metal and substituent thereby avoids the significant energy penalty associated with copper's putative change from dIo to d8 and back again across the conjugate addition pathway. The nature of the novel tetracoordinate CUI structures represented by 6 eliminates the need to consider Cu"' entities as viable intermediates in the conjugate addition process. Furthermore, they provide a basis for understanding the accelerating effects of additives on the r e a ~ t i o n . ~ ~ It , ~can ~,~~,~~-~ be confidently predicted that the tetracoordinate intermediates with appropriate substituents will, like [Cu(CF3)4]- and (CF3)2C U S C ( S ) N E ~ be ~ , observed, ~ ~ , ~ ~ isolated, and structurally characterized.
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Acknowledgment. I owe a debt of gratitude to Professors K. Morokuma, D. Liotta, and A. Padwa, Emory University, for generous support during my sabbatical at the Emerson Center. Dr. S. Bertz (Complexity Study Center), as always, offered stimulating counterpoint. Supporting Information Available: Figure 2, showing optimized geometries of for 6b and 7b (1 page). This material is contained in many libraries on microfiche, immediately follows this article in the microfilm version of the journal, can be ordered from the ACS, and can be downloaded from the Intemet; see any current masthead page for ordering information and Intemet access instructions. JA95 1825P (23) During our preparation of this paper, Dorigo, Wanner, and Schleyer reported calculations for 4 and water-solvated analo s of 6 and 7.9eIn addition to arguing "Evidence for the Existence of CUI Intermediates in ... Reactions with Dialkylcuprates", the work pictures structures at variance with those for square-planar 6b and 7b (Figure 2). In particular, the water analog of 6b places the protons in the molecular plane, while the analog of 7b depicts a nonplanar transition state. Both of these outcomes would appear to be artifacts of geometry-optimizing the oxy solvent with OH bonds in close proximity to carbanionic centers. (24) Snyder, J. P. Angew. Chem., Inr. Ed. Engl. 1995,34, 80; 1995,34, 986. (25) Willert-Porada, M. A.; Burton, D. J.; Baenziger, N. C. J. Chem. Soc., Chem. Commun. 1989, 1633. (26) This complex enjoys a CUIelectronic distribution analogous to those in Scheme 1 when subjected to NPA analysis: Snyder, J. P., unpublished.
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